R Date and Time Functions
R has serveral date and time related functions.
date() functions returns a date without time
as character string.
Sys.time() returns the system's date and time as a
POSIXct object respectively.
 "Fri Jan 04 17:38:05 2013"
 "2013-01-04 17:47:39 EST"
 "POSIXct" "POSIXt"
POSIXctcontains seconds from 1970.
POSIXltis a list, contains:
sec, 0-61: seconds
min, 0-59: minutes
hour 0-23: hours
mday 1-31: day of the month
mon 0-11: months after the first of the year
year: years since 1900
wday, 0-6: day of the week
yday, 0-365: day of the year
isdst: Daylight savings time flag
>x <- "19:18:05" >y <- strptime(x,"%H:%M:%S") >y
 "2013-01-04 19:18:05"
 "POSIXlt" "POSIXt"
R date time format:
|%a||Abbreviated weekday name in the current locale. (Also matches full name on input.)|
|%A||Full weekday name in the current locale. (Also matches abbreviated name on input.)|
|%b||Abbreviated month name in the current locale. (Also matches full name on input.)|
|%B||Full month name in the current locale. (Also matches abbreviated name on input.)|
|%c||Date and time. Locale-specific on output, "%a %b %e %H:%M:%S %Y" on input.|
|%d||Day of the month as decimal number (01-31).|
|%H||Hours as decimal number (00-23). As a special exception times such as 24:00:00 are accepted for input, since ISO 8601 allows these.|
|%I||Hours as decimal number (01-12).|
|%j||Day of year as decimal number (001-366).|
|%m||Month as decimal number (01-12).|
|%M||Minute as decimal number (00-59).|
|%p||AM/PM indicator in the locale. Used in conjunction with %I and not with %H. An empty string in some locales.|
|%S||Second as decimal number (00-61), allowing for up to two leap-seconds (but POSIX-compliant implementations will ignore leap seconds).|
|%U||Week of the year as decimal number (00-53) using Sunday as the first day 1 of the week (and typically with the first Sunday of the year as day 1 of week 1). The US convention.|
|%w||Weekday as decimal number (0-6, Sunday is 0).|
|%W||Week of the year as decimal number (00-53) using Monday as the first day of week (and typically with the first Monday of the year as day 1 of week 1). The UK convention.|
|%x||Date. Locale-specific on output, "%y/%m/%d" on input.|
|%X||Time. Locale-specific on output, "%H:%M:%S" on input.|
|%y||Year without century (00-99). On input, values 00 to 68 are prefixed by 20 and 69 to 99 by 19 - that is the behaviour specified by the 2004 and 2008 POSIX standards, but they do also say "it is expected that in a future version the default century inferred from a 2-digit year will change".|
|%Y||Year with century. Note that whereas there was no zero in the original Gregorian calendar, ISO 8601:2004 defines it to be valid (interpreted as 1BC): see http://en.wikipedia.org/wiki/0_(year). Note that the standard also says that years before 1582 in its calendar should only be used with agreement of the parties involved.|
|%z||Signed offset in hours and minutes from UTC, so -0800 is 8 hours behind UTC.|
|%Z||(output only.) Time zone as a character string (empty if not available). Where leading zeros are shown they will be used on output but are optional on input. Note that when %z or %Z is used for output with an object with an assigned timezone an attempt is made to use the values for that timezone, but it is not guaranteed to succeed.|